Manufacturing chocolate balls has been known for decades and various technologies based on cylindrical rollers have been proposed such as the one described in EP923875 which describes a pair of parallel rollers having cavities defined on the outer cylindrical surfaces, a depositing device depositing a solidifiable liquid such as chocolate onto said cylindrical surfaces, thus providing two separate parts of food articles. By counter rotating the rollers, the two surfaces move towards one another and join the two separate parts into one food product. The whole process is based on the fact each individual food article is linked to the others by a film, or a web, made out of the same food material. It is also relying on the fact that, on cooling, chocolate contracts and readily de-moulds from the cavities.
Whereas this process is adequate for manufacturing chocolate balls from a liquid base, it is totally inappropriate for the manufacturing of ice cream products wherein the cavities would be filled by a frozen aerated product. The main obstacles against transferring this technology to ice cream products are that the cavities must be at a low enough temperature, otherwise the ice cream fed into these cavities will melt (at least at the surface), but if the cavities are below 0° C., at a temperature where ice starts to form, then the ice cream will stick to the surface and will not be easily ‘de-mouldable’.
Such problems are for example illustrated in JP62-91148 which attempts to propose a process for the manufacturing of ice balls while addressing the problem of ice sticking to the walls of the cavities and which can be described as follows.
When the corresponding cavities of the pair of rollers pass the point where they are the closest to one another, the frozen product in each cavity is not pressed hard enough against the contiguous product situated into the corresponding cavity on the other roller, when the cavities move again away from each other through the rotation of the rollers, the force linking the two half products is too weak in comparison with the adhesion between each half product and the cavity in which it is and thus it stays in the cavity and does not ‘de-mould’. JP62-91148 addresses this problem by i) heating one of the roller with an internal circulation of hot liquid, ii) by providing ejection mechanisms in each cavity of the other roller, and iii) providing excess material proud of the roller surface. These ejection mechanisms allow for the two half products to be pressed together while heating one roller allows for demoulding the product.
This technology does not constitute a practical solution for the problem raised by attempting to produce frozen aerated products using a pair rollers since the need to effectively melt the surface of each product to allow for its de-moulding raises unacceptable hygiene issues. In other respect, the ejection mechanisms situated in each and every cavity of a roller are extremely complex, difficult to maintain, and again constitute a hygiene hazard.
More recently it has been proposed process for the manufacturing of frozen aerated products comprising;                providing two separate forming elements,        providing at least one open cavity on a surface of each forming element,        providing filling devices for filling said cavities with a frozen aerated material,        filling two cavities, one on each forming element, with a frozen aerated material,wherein:        a. at least one of the cavities is filled with a frozen aerated product having an overrun of between 30% and 130%,        b. this product is then allowed to expand outside its cavity,        c. the two cavities are then moved opposite one another and the frozen aerated product in each cavity is pressed against the frozen aerated product in the other cavity.        d. The forming elements being cooled with liquid nitrogen and are at a temperature below −80° C., more preferably below −100° C.        
Preferably, the two separate forming elements are a pair of parallel rollers wherein each roller has a multiplicity of open cavities on its surface, the rollers counter-rotating so that respective cavities in the two forming elements lie opposite one another and the frozen aerated product in a cavity of a first roller is pressed against the frozen aerated product in an opposite cavity of a second roller.
Each roller is hollow with its cavity being partially filled with liquid nitrogen so as to ensure cooling of the rollers.
Whereas such an apparatus is perfectly satisfactory with regards to the quality of the moulding process, it consumes, though sheer evaporation of the liquid nitrogen, a huge amount of liquid nitrogen which is in fact not used in the cooling process. Typically, in operation, less than 5% of the liquid nitrogen is effectively used to cool the moulds
There is therefore a need for an apparatus which dramatically decreases the liquid nitrogen consumption and whereas thermal insulation could be seen as an obvious solution, the study leading to the present invention has shown that such a potential solution was full of problems due to the stress created by thermal deformation. In that respect, Von Mises stress are preferably be below 200 MPa (for a safety factor 2), more preferably below 100 MPa (for a safety factor 3) in all points of the roller.